The present invention relates to pattern inspections according to which object images obtained using lights, or laser beams, are compared with reference images, and based on comparison differences, minute pattern defects or substances are found. The present invention particularly relates to a pattern inspection apparatus, and a method therefor, that is appropriate for conducting visual inspections of semiconductor wafers, TFTs and photomasks.
A method disclosed in JP-A-05-264467 is a known conventional technique for comparing an object image to be inspected with a reference image, and for detecting pattern defects.
According to this method, inspection samples wherein a repeated pattern is regularly arranged are sequentially scanned by a line sensor, and the obtained image is compared with an image obtained with a delay equivalent to the repeated pattern pitch. An unmatched portion is detected as a pattern defect. This conventional inspection method will be described while employing, as an example, the visual inspection of a semiconductor wafer. As is shown in FIG. 6, multiple chips having the same pattern are arranged in a semiconductor wafer to be inspected. Each of these chips can be roughly classified into a memory mat portion 71 and a peripheral circuit portion 72, as is shown in FIG. 7. The memory mat portion 71 is a set of small, repeated patterns (cells), while the peripheral circuit portion 72 is basically a set of random patterns. Generally, the memory mat portion 71 has a high pattern density, and a dark image is obtained by a bright-field illumination optical system. However, since the peripheral circuit portion 72 has a low pattern density, a bright image is obtained.
During a conventional visual inspection, images located at the same positions in the peripheral circuit portions 72 of adjacent chips, e.g., images in areas 61 and 62, etc., are compared, and a difference in the two is detected as a defect. At this time, since the two images are not always aligned, due to the vibration of a stage or the tilting of an object, a position shift distance is calculated between an image obtained by a sensor and an image that has been delayed by a time equivalent to a repeated pattern pitch. Registration of the two images is performed based on the obtained position shift distance, and a difference between the images is calculated. When the difference is greater than a predetermined value, this is determined to be a defect, or when the difference is smaller, this is determined to be a non-defect.
Generally in the registration of the two images during the comparison processing, the edges of the images are employed as a set of information for calculation of the position shift distance, and the position shift distance is calculated so that misalignment of the patterns in the images is minimized. Actually, a method for using normalized cross-correlation or a method using a residual sum is proposed.